1. Field of the Invention
The present invention relates to an ink jet printing apparatus and method for carrying out printing using a gradation pattern for a systematic dither method and a dot pattern in which dot arrangement information is stored, as well as a program for this ink jet printing method.
2. Description of the Related Art
An ink jet printing apparatus prints an image on a printing medium by causing ink droplets to be ejected through ink ejection openings constituting nozzles in an ink jet print head (hereinafter referred to as a “print head”) so that the ink is attached to the printing medium. In a method of causing ink to be ejected from a print head, the ink is ejected through ejection openings by applying an electric signal to heating elements (electrothermal converters) installed near the respective ejection openings to change the state of the ink involving a rapid change in volume (generation of bubbles), thus exerting force based on this change in state. In another method of causing ink to be ejected from a print head, the ink is ejected through ejection openings by using piezoelectric elements (electromechanical converting elements) or the like to change the pressure of the ink on the basis of a mechanical change. Another printing process is a serial scan method of printing an image on a printing medium by repeating a printing operation of causing ink to be ejected from a print head, which is simultaneously moved in a main scanning direction and a conveying operation of conveying the printed medium by a predetermined amount in a sub-scanning direction crossing the main scanning direction.
With an ink jet printing apparatus based on the serial scan method using such a print head, a high-grade image can be printed at a high speed with little noise. Further, a plurality of ejection openings can be densely arranged in the print head in its sub-scanning direction. Thus, this ink jet printing apparatus has a large number of advantages; in spite of its small size, it can easily produce high-resolution printed images and not only monochrome images but also colored images regardless of the size of the printing medium. A print head of what is called a “multi-nozzle” can be provided by integrating together a plurality of ink ejection openings and channels constituting the nozzles so as to allow a plurality of printing elements to be integrally arranged. Further, to print a colored image, a plurality of print heads of a multi-nozzle type are used.
However, with the increased resolution of printed images, an enormous amount of data must be processed in the printing apparatus. Thus, with a print system composed of an image processing section and an ink jet printing section, the throughput of the whole system may decrease sharply because of the speed at which the image processing section processes data or the speed at which the image processing section transfers data to the ink jet printing section. Further, with the increased resolution of printed images, it is necessary to increase the capacity of memory required in the ink jet printing apparatus main body in order to store data. This may increase the cost of the printing apparatus.
Thus, in the recent ink jet printing apparatuses, the image processing section transfers relatively-low-resolution image data subjected to a multivalued quantization process, to the ink jet printing section. The ink jet printing section then carries out printing (dot matrix printing) by expanding the received quantized low-resolution image data into a predetermined matrix.
A systematic dither method is a typical one of the multivalued quantization methods for the image processing section, i.e. the conversions into n values (n≧3). The systematic dither method uses dither matrices in which thresholds irrelevant to an input image are regularly arranged and repeatedly arranges dither matrices in a vertical direction and a horizontal direction. Then, the gradation of the input images is expressed by n values (n≧3) on the basis of the input image and the thresholds of the corresponding dither matrix. With the common systematic dither process, the regular arrangement (hereinafter also referred to as the “gradation pattern”) of the thresholds is of a dot distribution type or a dot concentration type.
FIG. 13 shows gradation patterns of a typical dot distribution type (Beyer type) which represent 256 gradations. These gradations correspond to an 8×8 matrix. The ink jet printing section has a dot matrix corresponding to a gradation value composed of n (n≧3) values. A plurality of predetermined dot patterns are stored in this dot matrix. FIGS. 14A to 14C show dot patterns set in a dot matrix and each composed of 2×2 pixels in association with a gradation value composed of five values “0” to “4”.
For example, it is assumed that image data is printed by allowing the image processing section to execute a multivalued quantization process to quantize the image data into nine values (4 bits) at a resolution of 300 DPI (horizontal)×300 DPI (vertical) and allowing the ink jet printing section to expand the quantized image data into a 4×2 matrix of resolution 1,200 DPI (horizontal)×600 DPI (vertical). In this case, the image processing section executes a quantizing process with a relatively low resolution of 300 DPI. This reduces loads on the image processing section compared to a quantizing process with a relatively high resolution of 1,200 DPI. Further, one piece of 4-bit image data of resolution 300 DPI corresponds to four pieces of 1-bit image data of resolution 600×600 DPI or to eight pieces of 1-bit image data of resolution 1,200×600 DPI. Thus, the amount of data transferred from the image processing section to the ink jet printing section is half of the amount of data transferred if the ink jet printing section expands the data into a matrix of resolution 600×600 DPI.
Further, Patent Document 1 describes an arrangement in which as a dot pattern with a gradation value of “1” such as the one shown in FIG. 14B, a plurality of dot patterns are provided which are different in the position of each dot in a 2×2 dot matrix so that the dot pattern used can be sequentially changed. Similarly, a plurality of dot patterns with a gradation value “2” or “3” such as those shown in FIGS. 14C and 14D are provided so that the dot pattern used can be sequentially changed. The dot pattern used may be sequentially changed during a single printing scan operation or in accordance with the printed position of the image or may be randomly changed.
[Patent Document 1] Japanese Patent Application Laying-open No. 9-046522 (1997)
However, for the conventional ink jet printing apparatus based on the serial scan method using the systematic dither process, when examining the durability of the print head achieved if images are constantly printed over a long period, the inventors found that an adverse effect on the durability appears periodically in the plurality of nozzles in the print head.
With reference to the accompanying drawings, description will be given of the periodicity of the adverse effect on the nozzles.
With an ink jet printing apparatus based on, for example, a method of utilizing thermal energy to bubble ink to eject ink droplets, images may be degraded that are printed using those of the plurality of nozzles of a print head capable of ejecting ink that are particularly frequently used to eject the ink for printing over a long period. This may be because dyes or impurities in the ink are thermally solidified and deposited on heater surfaces of electrothermal converters used to supply thermal energy to the ink.
In the above conventional example, if the systematic dither method is used to print images constantly over a long period, the nozzles in the print head are not uniformly degraded. As shown in FIG. 15, degraded nozzles through which, for example, ink cannot properly eject appear periodically in the direction in which the nozzles are arranged. This is because the nozzles corresponding to the fixed periodic pattern are used (ink ejection) more frequently than the others. In FIG. 15, the period corresponds to 16 nozzles in turn corresponding to the size of the gradation patterns based on the systematic dither method.
This is because the gradation patterns based on the systematic dither method are repeatedly used in the vertical and horizontal directions within the area in which image data is present, so that the gradation pattern is fixed with respect to the image data. Another cause is that before and after the print head carries out a printing scan in the horizontal direction (main scanning direction), the distance the print head and a printed medium are relatively moved in the vertical direction (sub-scanning direction) becomes an integral multiple of the size of the gradation pattern (or the size of the gradation pattern becomes an integral multiple of the distance the print head is moved relative to the printed medium in the vertical direction during printing scans), so that there is a fixed relationship between the gradation pattern and the positions of the nozzles in the print head.
Furthermore, with respect to the image data expressed by n values (n≧3) on the basis of the gradation patterns based on the systematic dither method, the dot pattern for the corresponding dot matrix is used. The nozzles based on this dot pattern are used (ink ejection) more frequently than the others.
For example, FIGS. 10B to 10F show the use (ink ejection) frequency of the nozzles used to print images of half tone densities (duty: 5, 10, 15, and 25%) using the gradation patterns shown in FIG. 13 and the dot patterns shown in FIG. 14. In this case, in order to emphasize the characteristics of the problem, dot patterns are used in which one dot is arranged in a 2×2 dot matrix as shown in FIG. 10A. The use (ink ejection) frequency of the nozzles is periodical on the basis of the size of the gradation patterns based on the systematic dither method, shown in FIG. 13. Thus, the use (ink ejection) frequency of the nozzles shown in FIGS. 10B to 10F corresponds to the number of times (probability) those nozzles are used to print an area of 16×16 dots using the gradation patterns (8×8) in FIG. 13. If, for example, an image of duty 5% is to be printed as shown in FIG. 10B, nozzles 1 and 9 are used twice due to the relationship between the gradation patterns (8×8) in FIG. 13 and the printing area (16×16 dots) and the operated nozzles of the print head as shown in FIG. 17.
The conventional example shown in FIGS. 10B to 10F uses only the pattern in which one dot is located in the upper left of a 2×2 dot matrix as shown in FIG. 10A. Thus, even with an increased halftone density, only every other nozzle is uniformly used as shown in FIG. 10F. Only the nozzles with the odd numbers are used. As a result, the degradation of particular nozzles used more frequently is markedly reflected in a printed image. The degraded nozzles may cause, for example, a variation in the ink ejection direction, a variation in the amount of ink ejected, or even the inability to eject ink.
Further, when degraded nozzles through which, for example, ink cannot properly eject appear significantly periodically, one of a nozzle number L and a nozzle number K is an integral multiple of the other, i.e. the following relationship is established: K=L×a (a is a natural number) or L=K×b. K is the number of nozzles in the print head corresponding to the amount by which a printed medium is conveyed while the print head carries out forward and backward printing scans. Specifically, in an ink jet printing apparatus based on the serial scan method of repeating a printing scan in the main scanning direction of a print head and the conveyance of a printed medium in the sub-scanning direction (along the direction in which nozzles are arranged), K is the number of nozzles in the print head corresponding to the amount by which the printed medium is conveyed. Further L is the size of the gradation patterns based on the systematic dither method in the nozzle arrangement direction and corresponds to the number of nozzles.
For example, if a printing operation is performed by using a print head in which 1,280 nozzles are arranged along the sub-scanning direction and intermittently conveying a printed medium by an amount corresponding to the 1,280 nozzles, then K=1,280. In this case, if a gradation pattern such as those shown in FIG. 13 is used, i.e. if the size of the gradation pattern corresponding to the number of nozzles in the nozzle arrangement direction is 16 as shown in FIG. 17, then K=L×80. That is, degraded nozzles appear significantly periodically as shown in FIG. 15. In contrast, if the size L of the gradation pattern is larger than K, degraded nozzles appear similarly periodically even when L=K×b (b is a natural number).